Low work function metal complexes and uses thereof

ABSTRACT

The present invention provides conductive metal-ligand coordination complexes that are useful in a variety of electronic devices. For example, such complexes are useful in organic light emitting devices composed of one or more layers of organic material between two conductors. The use of metal-ligand coordination complexes of the present invention as the cathode, replaces the more typically employed reactive metals, which function as the electron injecting contact, and provides for improved or longer-lived devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of PCT/US2003/025150, filed12 Aug. 2003, now International Publication No. WO 2004/015746 A2,published 19 Feb. 2004, which claims the priority benefit to U.S.Provisional Application No. 60/403,113, filed 12 Aug. 2002, both ofwhich are incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant No.CHE-0139637 awarded by National Science Foundation.

FIELD OF THE INVENTION

This invention relates to heteroaryl-metal coordination complexes havinga low work function and methods for use thereof.

BACKGROUND OF THE INVENTION

Organic light emitting devices (i.e., OLEDs) produce light emission uponapplication of current. See, for example, Tang and VanSlyke, Appl. Phys.Lett., 51, (1987), p. 913-915; Burroughs, et al., Nature, 347, (1990),p. 539-541. In a simplest form, OLEDs are comprised of one or more thinlayers of organic light emissive materials sandwiched between twoconductors (i.e., electrodes). An applied potential across theelectrodes injects electrons and holes from the cathode and anode,respectively, ultimately producing an excited state light emissivematerial. Light is produced by photon emission by the excited lightemissive material as it returns to the ground state. Preferably, thebarrier to electron or hole injection at both electrode interfacesshould be kept low to reduce power consumption.

A wide variety of organic materials have been used to transport charge(electron or hole) from the electrode to the light emissive material.However, the requirements for the cathode and anode are such thatrelatively few choices are available. Specifically, it is important thatthe work functions of the electrodes are well matched to the appropriateenergy levels in the organic charge transport materials.

Conventional efforts have focused on OLEDs usingtris(8-hydroxyquinoline) aluminum(III) complex (Alq₃) as the lightemissive and electron transport layer, and a triarylamine compound asthe hole transport medium. The advantage of this approach lies in partin the emission of light at or near the interface between the twoorganic layers.

Without being bound by any theory, it is believed that a low barrier forinjection should be provided when the Fermi energy of the cathode isclosely matched to the lowest unoccupied molecular orbital (LUMO) energyof the light emitting (or electron transport) layer. To this end, lowwork function (Φ) metals such as magnesium, calcium, and aluminum, ortheir alloys with silver are the most commonly used materials. See, forexample, Tang et al., Appl. Phys. Lett., 1987, 51, 913; Burrows et al.,Appl. Phys. Lett., 1994, 64, 2285; and Matsumura et al., J. Appl. Phys.,1996, 79, 264.

Recently, thin layers of insulators and wide band gap semiconductorsbetween the conducting cathode and the active organic material have beenutilized. Specifically, inorganic insulators such as LiF, Li₂O, MgF₂,and MgO under aluminum metal have been widely used, (see Hung et al.,Appl. Phys. Lett., 1997, 70, 152; Jabbour et al., Appl. Phys. Lett.,1997, 71, 1762; Matsumura et al., Appl. Phys. Lett., 1998, 73, 2872; andLee, Synth. Met., 1997, 91, 125) and copper phthalocyanine (CuPc) hasbeen paired with the high work function conducting metal oxide indiumtin oxide (ITO) (Parthasarathy et al., Appl. Phys. Lett., 1998, 72,2138) to produce a reasonably efficient transparent OLED. Doping oflithium metal into Alq₃ by codeposition has also been reported. Kido etal., Appl. Phys. Lett., 1998, 73, 2866.

Often the anode is composed of a transparent conducting oxide such asindium-tin oxide (ITO), the high work function of which is well matchedin energy to the highest occupied molecular orbital (or valance band) inthe light-emitting material. The cathode generally contains a low workfunction metal, for example, calcium or magnesium, such that the barrierfor injection of electrons into the lowest unoccupied molecular orbital(or conduction band) of the organic layer, e.g., light-emittingmaterial, is as small as possible. To provide more stability, thesemetals are typically alloyed with or covered by silver or gold.

The failure mechanism for such light-emitting devices is generallyassociated with degradation of the cathode. Tang et al., Appl. Phys.Lett., 1987, 51, 913-915. Other problems including oxide buildup andlocalized heating have also been attributed to this interface. See, forexample, Choung et al., Appl. Phys. Lett., 1998, 72, 2689-2691; andBurroughs, et al., Nature, 1990, 347, 539-541.

Other approaches have been shown to improve performance at thecathode/organic interface. One method involves placing a very thin layerof an insulating salt such as LiF, MgO (Hung et al., Appl. Phys. Lett.,1997, 70, 152-154), MgF₂ (Lee, Synth. Met., 1997, 91, 125-127) or Al₂O₃(Tang et al., Appl. Phys. Lett., 1997, 71, 2560-2562) separating theorganic material and an aluminum metal covering. Devices of this typeare more efficient and have longer operating lifetime than previousmodels.

A series of low Φ conducting polymers have also been reported, one ofwhich was used in conjunction with ITO as a cathode in an “invertedtype” OLED. Bloom et al., J. Am. Chem. Soc., 2001, 123, 9436. Thematerials were composed of redox active substituted transition-metaldiimine complexes, which as thin films were thermally polymerizable.Electrochemical reduction of the polymers yielded conductive films withwork functions (which could be predicted from cyclic voltammetry of themonomers) from 3.7 to 3.0 eV. An OLED consisting of the layersgold/TPD/Alq₃/polymer/ITO was reported where gold was the anode. (TPD isa commonly used organic hole transport material). While this type ofdevice produces light under a moderate voltage bias, the performance wasnot optimal.

The latter strategy involves the use of a series of conducting polymershaving a low work function, one of which was used as a cathode materialfor an OLED. Without being bound by any theory, it is believed thatcathodes comprising a conductive organic material would be inherentlymore compatible with the light-emitting (i.e., luminescent) materialsthan the metals that are typically employed. There are inherentlimitations of this system that make it of limited utility for theconstruction of devices, however, including the necessity ofpost-polymerization electrochemical processing of the polymers.

In view of the problems associated with current OLEDs, it is evidentthat there is a need for other low work function materials that can beused in a variety of electronic devices, such as OLEDs.

SUMMARY OF THE INVENTION

The present invention provides heteroaryl-metal coordination complexesand methods for use thereof. In one embodiment, the heteroaryl-metalcoordination complexes can be electrochemically or chemically reduced,preferably at a potential more negative than−−0.5 volts vs. a saturatedcalomel electrode (SCE), to produce a material that is electronicallyconducting in the solid state.

In another embodiment, the reduced heteroaryl-metal coordinationcomplexes can be deposited, e.g., by vacuum vapor deposition, to formelectronically conducting thin films. These reduced materials have lowwork functions, and can thus function as electrodes, preferablycathodes, for OLEDs, where they can be in contact with any suitableconductive material to provide a stable and rugged electrical contact.

The energies of the relevant reductions in a given heteroaryl-metalcoordination complex, and therefore the work function of a given redoxform or redox forms in the solid state, can be controlled by syntheticmodification of the starting material. In this manner, an electrode canbe essentially tailor made with the desired work function to match anactive organic layer of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of Current vs. Voltage plots for Example 1-3; and

FIG. 2 shows a graph of Light intensity vs. Voltage plots for Example1-3.

FIG. 3 shows a cyclic voltammetry graph of two different Cr complexes.

FIG. 4A is a XPS survey spectrum of vapor deposited Ru(terpy)₂ ⁰.

FIG. 4B is a high resolution XPS spectra of C, Ru, and N regions ofRu(terpy)₂ ⁰.

FIG. 5 shows UPS and XPS spectra binding energy graph of Ru(terpy)₂ ⁰.

FIG. 6 is a graph of current vs. bias and light intensity vs. bias of a[Ru(terpy)₂]⁰/Ag cathode.

FIG. 7 is a graph showing performance of four OLEDs with cathodes ofvarying heteroaryl-metal coordination complexes of the present inventionand covering metal layers.

FIG. 8A is a Current (I) vs. Voltage (V) graph of a “hole-only” devicewith silver, along with the analogous OLED from FIG. 7.

FIG. 8B is a Current (I) vs. Voltage (V) graph of a “hole-only” devicewith gold, along with the analogous OLED from FIG. 7.

FIG. 9 is a performance graph of OLEDs with different thickness of[Cr(bpy)₃]⁰ in the [Cr(bpy)₃]⁰/Ag cathode.

FIG. 10 is a graph showing current-voltage performance of an OLED ofconstruction Ag/[Ru(terpy)₂]⁰/Alq₃/TPD/PEDOT-PSS/ITO (as shown in FIG.7) and a fit to the equation I ∝ e^(cV).

DETAILED DESCRIPTION OF THE INVENTION

Abbreviations

Unless the context requires otherwise, the following abbreviations areused throughout this application.

-   -   LWOM=low work function organic materials    -   ITO=indium-tin oxide    -   TMB=4,4′,5,5′-tetramethyl-2,2′-bipyridine    -   bpy=2,2′-bipyridine    -   terpy=2,2′:6′,2″-terpyridine    -   TPD=N,N′-bis(3-methylphenyl-N,N′-diphenylbenzidine)    -   PEDOT-PSS=poly(styrenesulfonate)-poly(2,3-dihydrothieno-[3,4-b]-1,4-dioxin)    -   NHE=normal hydrogen electrode    -   SSCE=saturated sodium chloride calomel    -   HBEC=high binding energy cutoff        Definitions

Alkyl groups for the present invention are aliphatic hydrocarbons whichcan be straight or branched chain groups. Alkyl groups optionally can besubstituted with one or more substituents, such as a halogen, aryl,hydroxy, amino, thio, alkoxy, carboxy, or oxo. There may be optionallyinserted along the alkyl group one or more oxygen, sulfur, orsubstituted or unsubstituted nitrogen atoms.

The term “cycloalkyl” refers to a monovalent mono-, di-, tri-, ortetra-cyclic hydrocarbon moiety. The cycloalkyl can be optionallysubstituted independently with one more of the following groups: alkyl,halide, hydroxyl, amino, carboxyl, aryl, or heteroaryl. In addition, oneor more ring carbon atoms of the cycloalkyl group can be replaced with aheteroatom such as nitrogen, oxygen, sulfur, phosphorous, or acombination thereof.

The term “aryl” refers to an aromatic ring moiety, such as mono-, bi-,or tri-cyclic aromatic ring species. Aryl groups can be substituted withone or more substituents described above for the alkyl group.

The term “heteroaryl” means a monovalent mono-, bi-, tri-, ortetera-cyclic aromatic moiety containing one or more heteroatomsselected from N, O, P, S, or combinations thereof, the remaining ringatoms being C. The heteroaryl ring can be optionally substitutedindependently with one or more substituents described above for thealkyl group.

An “ester” group is of the form —CO(OR,) where R_(x) can be alkyl, aryl,or heteroaryl. An “amide” function is comprised of —CO(NR_(y)R_(z))where R_(y) and R_(z) can be the same or different and are defined thesame as R_(x).

Unless otherwise stated, work function values given in the presentapplication are predicted work function values derived fromsolution-phase cyclic voltammetry of the parent complexes.

General Overview

The present invention provides metal-ligand coordination complexes thathave a low work function, preferably 3.5 eV or less. Such complexes areuseful in a variety of electronic devices where a low power consumptionin transfer of electrons from one interface to another is desired.Preferably, metal-ligand coordination complexes of the present inventioncomprise redox active metal complexes. In this manner, metal-ligandcoordination complexes can be isolated in the formally zero-chargedstate, for example, via reductive electrocrystallization. In onespecific embodiment, the reduced (i.e., formally zero-charged state)metal-ligand coordination complexes are used as electrode, e.g.,cathode, materials in organic light-emitting devices.

In one particular embodiment, metal-ligand coordination complexes of thepresent invention are thermally evaporated or vacuum vapor deposited onto a substrate to form conducting thin films with low work functions.Such deposition processes provide a variety of advantages over theconventional polymers in OLED applications. For example, by eliminatingthe necessity of postpolymerization electrochemical processing,metal-ligand coordination complexes of the present invention are easierto use and allow a much greater variety of device architectures, and/ormore pure material deposition.

Metal-Ligand Coordination Complex

In one aspect of the present invention, at least one ligand ofmetal-ligand coordination complex is a heteroaryl moiety. Suchmetal-ligand coordination complexes are also referred herein asheteroaryl-metal coordination complexes. A particularly preferredheteroaryl moiety is polypyridyl, phenanthroline, or a derivativethereof, with polypyridyl or a derivative thereof being an especiallypreferred heteroaryl moiety. Such complexes are useful in a variety ofelectronic applications, including as electrodes, preferably cathodes,in OLEDs.

In one particular embodiment, the heteroaryl-metal coordination complexis of the formula:[M-(L)_(a)]_(m)Y_(n)  Iwhere M is a metal, L is a ligand, Y is a counterion, a is an integerfrom 1 to 6 inclusive, and m and n are absolute value of the oxidationstate of Y and [M-(L)_(a)], respectively.

Referring to Formula I, the subscript a represents the number of ligandspresent on the metal M. For example, when M contains six ligands and Lis a bidentate ligand, a is 3. For the same M requiring 6 ligands, if Lis a tridentate ligand, then a is 2.

In Formula I, each ligand, L, is independently a mono- or polydentateligand, preferably a mono- or polydentate heteroaryl moiety containingone or more, preferably two or three, coordinating heteroatoms thatcoordinate with the metal M. Suitable heteroatoms include N, O andS(O)_(x), where x is 0, 1 or 2. A particularly preferred heteroatom is Nor O, with N being an especially preferred heteroatom. In one particularembodiment, the heteroaryl moiety is preferably comprised of two or moreheteroaryl groups that are fused or are covalently bonded together.While each L can be independent of the other, it is preferred that allthe ligands, L, be of the same moiety.

In one particular embodiment, L is a heteroaryl moiety selected frompolypyridyl, phenanthroline, and a derivative thereof.

In one embodiment, each L is independently optionally substituted2,2′-bipyridyl, optionally substituted 1,10-phenanthroline, optionallysubstituted 2,2′,6′,2″-terpyridyl or a derivative thereof. In oneparticular embodiment, all the L's are same.

In another embodiment, L is polypyridyl. A particularly preferredpolypyridyl is selected from 4,4′,5,5′-tetramethyl-2,2′-bipyridyl (TMB);2,2′-bipyridyl (bpy); and 2,2′,6′,2″-terpyridyl (terpy).

Exemplary polypyridyl moieties of L include the following chemicalstructures:

where each R is independently a substituent known to one skilled in theart, preferably a substituent selected from the group consisting ofhydrogen, alkyl, cycloalkyl, aryl, heteroaryl, ester, amide, halide, andan electron donating group. Electron donating groups are well known toone skilled in the art and include substituents such as hydroxy, alkyl,cycloalkyl, aryl, alkoxy, aryloxy, cycloalkyloxy, amino, mono- anddi-alkyl amino. The substituents R may all be the same or different.

In one particular embodiment, L is a moiety of the formula:

where each of the substituents R is independently those defined hereinand the subscript indicates the position of the substituent.

A particularly group of preferred ligand A(I) are those whereR₄=R′₄=R″₄=R₅=R″₅=H; and R₄=R′₄=R″₄=methyl and R₅=R′₅=H. A particularlygroup of preferred ligand A(II) are those where R₄=R′₄=R₅=R′₅=methyl;and R₄=R′₄=R₅ =R′₅ =H.

Referring again to Formula I, M is a metal, which may or may not itselfexist in more than one stable oxidation state. Preferably, M is atransition metal. More preferably, M is a metal selected from Ru, Cr,Fe, Zn, Co, Mn, Cu, Os, Rh, and Ni. A particularly preferred M is Ru,Cr, or Fe, with Ru and Cr being an especially preferred M group.

The variable Y in Formula I represents a counterion, e.g., either anionor cation, necessary to maintain an overall electroneutrality of themetal-ligand coordination complex. While Y can sometimes affect thesolubility or other properties of the complex, the exact nature of Y isin general not critical.

When the mean formal charge on [M-(L)_(a)] in the solid material is notzero and is an integer, the variables m and n are absolute values of theoxidation state of the counterion (or sum of the charges on multiplecounterions) and the metal-ligand complex, respectively. When the meanformal charge on [M-(L)_(a)] in the solid material is zero no Y ispresent and the formula of the metal-ligand complex is simply M-(L)_(a),where M, L and a are those defined herein. When the material ismixed-valent such that the mean charge per metal complex is anon-integer, then n=1 and m is the absolute value of the quantityobtained by dividing the oxidation state of Y by the mean charge per[M-(L)_(a)]. An example of such a mixed-valent material is onecontaining equal molar amounts of [M-(L)_(a)] with a formal charge of 2+and [M-(L)_(a)] with a formal charge of 1+. The mean charge on[M-(L)_(a)] in such a material would thus be 1.5+. If Y is a monoatomicanion with an oxidation state of 2−, then n=1 and m=|(1.5)/(−2)|=0.75.When counterion Y is present, preferred counter anions are PF₆ ⁻, ClO₄³¹ , BF₄ ⁻, and NO₃ ⁻. Preferred counter cations are Li⁺, Na⁺, K⁺, Cs⁺,Rb⁺, and NR₄ ⁺ where R is a straight or branched chain alkyl containing1 to 8 carbon atoms.

In one particularly preferred embodiment, the complex is not charged andthus no counterion is present and the formula of the complex is simplyM-(L)_(a). Such neutral metal-ligand coordination complex can beobtained by reducing a corresponding positively charged metal-ligandcoordination complex as discussed below.

In one embodiment, the metal-ligand complex of the present invention isselected from compounds of the formula: [Ru(Ligand A(I))₂]_(m)Y_(n);[Fe(Ligand A(I))₂]_(m)Y_(n); [Cr(Ligand A(II))₃]_(m)Y_(n); and[Cr(Ligand A(III))₃]_(m)Y_(n).

Preferably, the metal-ligand complex is in formally zero-charged state,i.e., uncharged, and Y is not present (i.e., m is 1 and n is zero).

In another embodiment, the average charge per metal-ligand coordinationcomplex is not specified and may be positive, zero, or negative.Conductive films of metal-ligand coordination complex can be preparedfrom solutions of metal-ligand coordination complex by spin coating,spray coating, dip coating, or any other conventional methods known toone skilled in the art of forming a thin film on a substrate surface,for example, vapor deposition including vacuum vapor deposition of asolid metal-ligand coordination complex. The film can be applied firstto a conductor or can be applied to materials, e.g., the organic orlight-emitting layer, directly. The [M-(L)_(a)] film can serve as thecathode alone or it can be contacted with other conductors orinsulators.

When the metal-ligand coordination complexes are electricallynon-neutral, i.e., the metal is in the oxidation state of 1+ or higher,such metal-ligand coordination complexes can be reduced, e.g., byelectrochemical or chemical reduction, to produce metal-ligandcoordination complexes of the formally zero-charged state. Suchelectronically neutral metal-ligand coordination complexes areelectronically conductive. In one particular instance, theelectroconductivity of such neutral metal-ligand coordination complexwas found to have resistivity, ρ, of about 1×10³ Ω·cm.

Without being bound by any theory, the conductivity and low workfunctions can be explained by considering the consequences of theproximal redox processes, as predicted by the Nernst equation. Becauseof thermal energy, at least three oxidation states (1+, 0, and 1−, wherethe charged species are present in equal numbers) are present inmoderate concentration at room temperature in the reduced, i.e.,formally zero-charged, state of metal-ligand coordination films.Furthermore, because the LUMO energy of the free ligands can becontrolled by synthetic alterations, e.g., different substituents orheteroaryl moieties, the work function, Φ, of the metal-ligandcoordination complexes can be modified.

As stated above, the Φ can be estimated or predicted by cyclicvoltammetry of the metal-ligand coordination complex in solution fromthe average of the E_(1/2) of the 1+/0 and 0/1− couples. In general, thesame factors that apply to the conductivity and Φ of the previouslydescribed polymers (Bloom et al., J. Am. Chem. Soc. 2001, 123, 9436)also apply to solids composed of neutral metal-ligand coordinationcomplexes with similar redox properties.

Metal-ligand coordination complexes of the present invention are redoxactive and are believed to possess numerous oxidation states with smallvoltage separations. As stated above, neutral complexes can be producedby electrochemical or chemical reduction of the complex, by adding anumber of electrons equal to the initial positive charge. Also, byvirtue of a lack of formal charge, isolation of the zero-valent form isoften simple because of the different solubility than the chargedspecies. For example, the solid-state compounds [Ru(bpy)₃]⁰ and[Ru(terpy)₂]⁰ can be prepared by reductive electrocrystallization fromthe 2+ species.

Additional objects, advantages, and novel features of this inventionwill become apparent to those skilled in the art upon examination of thefollowing examples thereof, which are not intended to be limiting.

EXAMPLES

General

Acetonitrile (Aldrich Optima grade) was stored over 4 Å molecularsieves, and distilled from CaH₂. Ammonium hexafluorophosphate (NH₄ ⁺PF₆⁻) was supplied by Elf Atochem and tetra-n-butylammoniumhexafluorophosphate (TBA⁺PF₆ ⁻) electrolyte was prepared as previouslydescribed by the present inventor. Elliott et al., J. Electroanal. Chem.1986, 197, 219. The ligand 4,4′,5,5′-tetramethyl-2,2′-bipyridine (TMB)was produced by a coupling reaction of 3,4-lutidine (Aldrich) over Pd onC, followed by recrystallization from ethyl acetate. 2,2′-Bipyridine(bpy) was purchased from Baker, and 2,2′:6′,2″-terpyridine (terpy) fromAldrich, and both were used without further purification. Alq₃ and TPD,from Aldrich, were purified by vacuum train sublimation with Argon gasflow at 330° C. and 270° C., respectively. Chromic chloride was suppliedby Fisher. Gold of purity 99.99% was purchased from Alfa Aesar, and99.9999% pure silver from Sargent-Welch. The conducting polymerdispersion,poly(styrenesulfonate)-poly(2,3-dihydrothieno-[3,4-b]-1,4-dioxin) 1.3 wt% in H₂O (PEDOT-PSS), was supplied by Aldrich. ITO, 4-8 Ω/sq. on glass,was purchased from Delta Technologies, Ltd.

Example 1

The complex [Ru(2,2′,6′,2″- terpyridine)₂]²⁺(PF₆ ⁻)₂ was synthesized asfollows. Terpyridine (100 mg, 0.429 mmol) in 10 mL ethylene glycol wasadded to Ru-(DMSO)₄Cl₂ (95 mg, 0.196 mmol) in 4 mL 1:1 methanol/water.The mixture was heated under N₂ via an oil bath held at 100° C. for 2.5h, producing a dark reddish solution which was then cooled to roomtemperature. Water was added to reach a total volume of 75 mL and NH₄⁺PF₆ ⁻was added whereupon a red/orange precipitate formed. The solid waswashed well with H₂O and recrystallized from methanol to yield brick redcrystals.

Inside an inert atmosphere glove box, the complex was electrochemicallyreduced in a three compartment bulk electrolysis cell at −2.00 V vsAg/Ag⁺ 0.1 M in DMSO in an CH₃CN electrolyte solution. Crystals of thereduced complex [Ru(2,2′,6′,2″-terpyridine)₂]⁰ formed on the workingelectrode and were dislodged and isolated. In this state, the complexcan be evaporated in a vacuum thermal deposition chamber, and is used inthe same manner as other standard depositable small molecule compoundsin OLEDs.

A patterned ITO substrate was cleaned by sonication in an aqueouscleaning solution and covered with a layer of the conducting polymerPEDOT-PSS, and was then placed in a vacuum deposition chamber forfurther processing. A thin layer of the organic hole transport materialTPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine) was thendeposited, followed by the emitting material tris(8-hydroxyquinoline)aluminum (III) complex, known as Alq₃. In this example, an approximately100 Å thick layer of the neutral complex [Ru(2,2′,6′,2″- terpyridine)₂]⁰was deposited over the Alq₃, followed by a covering layer of silver orgold metal. The layers in the device were thus as follows:M/[Ru(2,2′,6′,2″-terpyridine)₂]⁰/Alq₃/TPD/PEDOT-PSS/ITO, where M iseither Ag or Au.

OLEDs produced in the above manner were tested by simultaneouslymeasuring the current-voltage response and the resulting light poweroutput. The polarity of the voltage applied was positive on the ITOanode and negative to the silver/metal-complex cathode.

Example 2

The complex [Cr(2,2′-bipyridine)₃]³⁺(ClO₄ ⁻)₃ was synthesized using amodified literature procedure. See, for example, Pecsok et al., J. Am.Chem. Soc., 1950, 72, 189-193; and Baker et al., Inorg. Chem., 1965, 4,848-854.

Briefly, CrCl₃·6H₂O (1.33 g, 0.0050 mol) was refluxed under N₂ in 0.05 MHClO₄ (Mallinckrodt) over Al metal (Baker) to produce a blue/greensolution likely containing both Cr²⁺ and Cr³⁺. This solution was addedto a N₂-purged suspension of bpy (7.81 g, 0.050 mol) in aqueous HClO₄ ofpH 2. A black suspension was quickly formed, which slowly turned yellow(indicating formation of the [Cr(bpy)₃]³⁺ complex) upon bubbling with O₂for 2 h. The yellow solid was collected by filtration, washed well withwater, ethanol, and CH₂Cl₂ (to remove excess ligand), and finallyrecrystallized from water.

As described in Example 1, the complex was electrochemically reduced toform [Cr(2,2′-bipyridine)₃]⁰ in the nitrogen glove box at −1.90 V vsAg/Ag⁺ 0.1 MThis complex was then evaporated over Alq₃ and covered withmetal to form a cathode in an OLED and tested for performance.

Example 3

The complex [Cr(4,4′,5,5′-tetramethyl-2,2′-bipyridine)₃]³⁺(ClO₄ ⁻)₃ wassynthesized in a similar fashion as the bipyridine complex in Example 2.However, the procedure of Example 2 was changed slightly by using muchless TMB ligand (0.50 g), because in this case the large excess was notunnecessary, and 0.285 g chromic chloride. The resulting yellow solidwas washed with ethanol and boiling hot water.

As described in Example 1, the complex was electrochemically reduced to[Cr(4,4′,5,5′-tetramethyl-2,2′-bipyridine)₃]⁰ by holding the workingelectrode at −2.15 V vs Ag/Ag⁺ 0.1 M in DMSO. An OLED was constructed inthis example consisting of the layers:Ag/[Cr(TMB)₃]⁰/Alq₃/TPD/PEDOT-PSS/ITO, and tested for performance.

Current vs voltage and light output vs voltage charts for the OLEDsdescribed in Examples 1-3 are depicted in FIGS. 1 and 2. The data showsthat the devices produce light with an onset voltage of 3 to 4 Volts,and have an intense emission of up to 6×10⁻⁵ Watts for a device with anarea of ca. 0.3 cm².

Example 4

Cyclic voltammetry was performed in a Luggin capillary cell with aAg/Ag+0.1 M in dimethyl sulfoxide (DMSO) [0.41 V vs normal hydrogenelectrode (NHE)] reference electrode, Pt wire counter, and glassy carbonworking electrode, in 0.1 M TBA⁺PF₆ ⁻in CH₃CN electrolyte. The scan ratewas 50 mV/s. Electronics consisted of a P.A.R. model 173 Potentiostatand model 175 Programmer with output to a Yokogawa X/Y recorder.

Example 5

This example illustrates electrocrystallization process.

The three complexes were electrocrystallized following a modification ofa method described in the literature. Perez-Cordero et al., Helv. Chim.Acta, 1994, 77, 1222 and Pyo et al., Inorg. Chem., 1999, 38, 3337.

Briefly, a three-compartment bulk electrolysis cell was used in an inertatmosphere glovebox with the same electrolyte, reference, and counterelectrodes as above, and a Pt mesh working electrode (WE). To produce[Ru(terpy)₂]⁰, 50 mg of [Ru(terpy)₂]²⁺(PF₆ ⁻)₂ was added to the WEcompartment, and vigorously stirred. The WE was held at a constantpotential of −2.00 V (several hundred millivolts past the secondreduction as determined from cyclic voltammetry) until the currentdecayed from approximately 500 mA to less than 100 μA during the courseof several hours. The WE, which was covered with purple/black crystalsof the reduced complex, was disconnected from the potentiostat andremoved from the electrolysis solution. These crystals were dislodgedfrom the WE in fresh CH₃CN, collected on a fritted filter, and washedwith more acetonitrile. The solid was dried by passage of gloveboxatmosphere over the solid with a vacuum pump, and scraped into a boatfor thermal deposition. Crystals of [Cr(bpy)₃]⁰ and [Cr(TMB)₃]⁰ wereprepared in the same manner, but at controlled potentials of −1.90 V and−2.15 V vs Ag/Ag₊0.1 M in DMSO, respectively.

Example 6

Photoelectron Spectroscopy

All photoelectron spectroscopy work was performed in an Omicronmultiprobe UHV chamber (base pressure 5×10⁻¹¹ Torr) equipped with a VSWEA125 single-channel analyzer. A transfer rod assembly was used whichcould be moved into the glovebox for sample preparation, and the sampleisolated from the atmosphere behind a gate valve. This assembly wasaffixed to the entry chamber of the UHV system and pumped down to vacuumbefore introducing the sample into the analysis chamber. The X-raysource was the Mg Kα line at 1253.6 eV. The UV light source was a heliumarc lamp, providing a He(I) line at 21.22 eV and a He(II) line at 40.81eV. A −5.00 V bias was applied to the sample to separate thespectrometer and sample high binding energy cutoffs. Kinetic energyanalysis of electrons emitted normal to the sample was done using a 10eV pass energy. The spectrometer was calibrated with an Ar⁺ ionsputtered copper standard. A straight line was fit on the secondary edgeof the UPS He(I) spectrum (and the XPS spectrum). The intercept of thisline with the abscissa determines the high binding energy cutoff (HBEC).A value of 0.1 eV was subtracted from the HBEC to correct for spectrumbroadening due to thermal and analyzer effects. The work function wasdetermined by subtracting this value from the source energy of 21.22 eV.

Example 7

OLED Construction

After rinsing in ethyl acetate and isopropyl alcohol, patterned ITOsubstrates were cleaned in a laminar flow hood by successive sonicationin a 5% aqueous solution of VWR aquasonic cleaner followed by Milliporewater. For devices including PEDOT-PSS, the polymer suspension wasfiltered through a 0.2-μm cellulose acetate syringe filter (Nalgene) andspin coated onto an ITO substrate at approximately 1000 rpm using amodified commercial blender. The substrates were introduced first intothe glovebox and then the vacuum deposition chamber (Denton DV 502ATurbo model) that is directly interfaced with the glovebox. Organicmaterials and metals were sequentially deposited at pressures below3×10⁻⁶ Torr. The thickness of the various layers was measured by aLeybold Inficon quartz crystal microbalance and XTM-2 DepositionMonitor. Device testing was performed using a Kiethley 2400 Sourcemeterand Newport 1830-C Optical Power Meter with 818-SL Photodiode detectordriven by LabVIEW 6.0 Software.

Results and Discussion

From cyclic voltammetry (FIG. 3 and literature, Morris et al., J.Electroanal. Chem., 1983, 149, 115), the E_(1/2) potentials for thereductions along with calculated E_(F) and Φ, of [Ru(terpy)₂]⁰,[Cr(bpy)₃]⁰, and [Cr(TMB)₃]⁰ are presented in Table 1 below.

TABLE 1 E_(1/2) Potentials for Reductions of Metal Complexes. Conditionsfor Cyclic Voltammetry of Cr Complexes: Electrolyte, 0.1 M TBAPF₆ inCH₃CN; WE, Glassy C; CE., Pt Wire; Scan Rate, 100 mV/s E_(1/2) vs Ag/Ag⁺0.1 M in DMSO, V E_(1/2) vs NHE, V 3+/2+ 2+/1+ 1+/0 0/1− 1+/0 0/1− E_(F)calc., eV Φ calc., eV Ru(terpy)₂ −1.44* −1.69* −2.13* −1.28 −1.72 −1.503.10 Cr(bpy)₃ −0.48 −0.98 −1.56 −2.18 −1.15 −1.77 −1.46 3.14 Cr(TMB)₃−0.72 −1.25 −1.85 −2.46** −1.44 −2.05** −1.75 2.85 *Values from theliterature were originally reported vs SSCE (Morris et al., J.Electroanal. Chem., 1983, 149, 115) but have been converted for thistable. **Estimated by comparison with bpy complex.

As shown in Table 1, the TMB complex undergoes reductive processes atpotentials significantly more negative than does the bpy analogue. It isbelieved that this potential difference in the reductive process is dueto the electron-donating nature the methyl substituents on the ligands.Indeed, for this system, the fourth reduction (0/1−) is outside theelectrochemical window available in acetonitrile solvent and thereduction potential was estimated by comparison to the other Cr complex.

The reduction potential data were obtained using either a Ag/Ag⁺0.1 M inDMSO or saturated sodium chloride calomel (SSCE) reference electrode.Potentials are also reported relative to the standard electrode forelectrochemical data, the NHE, by subtracting the difference inpotential of the reference electrode used in the experiment from that ofthe NHE, 0.410 and 0.236 V, respectively. The E_(F) vs NHE is calculatedas the average of the half-wave potentials for the 1+/0 and 0/1−reductions, and the Φ is estimated by comparison of the energy of theNHE to the energy of a free electron in a vacuum, −4.60 eV.

X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) wereperformed on a 200-Å-thick film of [Ru(terpy)₂]⁰ thermally deposited onPt foil. XPS spectra (FIGS. 4A and 4B) revealed the presence of carbon,nitrogen, and ruthenium, as expected, as well as oxygen. Without beingbound by any theory, it is believed that the presence of oxygen is aresult of accidental contact of the reduced LWOM with O₂ during thetransfer process. The work function of the film was measured from theHBEC to be 3.32 and 3.38 eV, by XPS and UPS, respectively. See FIG. 5.These values are in good agreement with each other and are close to thevalue of 3.10 eV predicted from the electrochemical data. This Φ for[Ru(terpy)₂]⁰ provided by photoelectron spectroscopy is within the rangeof commonly used metals for OLED cathodes (by comparison, Ca metal has aΦ of 2.9 eV).

OLEDs were constructed using [Ru(terpy)₂]⁰ with the architectureAg/LWOM/Alq₃(400 Å)/TPD(400 Å)/ITO. Performance for a typical devicewith [Ru(terpy)₂]⁰ as the LWOM is shown in FIG. 6. Emission spectra forthese devices peak in the 520 nm region typical of Alq₃-based devices.

Another OLEDs were prepared including a conducting polymer interlayer(PEDOT-PSS) between the ITO anode and the TPD hole-transport material.This approach has been shown to improve device performance by reducingthe barrier to hole injection at the anode, resulting in increaseddevice lifetime and efficiency. See, for example, Troadec et al., Synth.Met. 2001, 124, 49 and Elschner et al., Synth. Met. 2000, 111-112, 139.Devices containing this additional layer for hole injection both passedmore current and produced more light at a given voltage, as shown inFIG. 7.

Analogous OLEDs were also made with gold, which has a very high Φ of 5.2eV. Devices with architecture Au/LWOM/Alq₃(400 Å)/TPD(200Å)/PEDOT-PSS/ITO performed very similarly to those with silver (see FIG.7). Accordingly, it appears that the nature of the metal contact is ofsecondary importance to the underlying LWOM. In contrast, when devicesare constructed in exactly the same manner, but omitting the LWOM layer,they emit only weakly and at high voltages with Ag metal cathodes andnot at all when Au is used. “Hole only” devices without Alq₃ also wereconstructed, consisting of the layers ITO/PEDOT-PSS/TPD (800Å)/Ru(terpy)₂ ⁰/Metal, with both Ag and Au metals. The current/voltageresponse of such devices (FIGS. 8A and 8B) with the two metals was verysimilar, and unlike in OLEDs including Alq₃, the current passed at biasgreater than about 3 V appeared to be essentially ohmic.

FIG. 7 also shows the performance of OLEDs with [Cr(TMB)₃]⁰ and[Cr(bpy)₃]⁰ as the cathode materials. As can be seen in FIG. 7, a deviceusing the TMB complex emits more light, while passing more current, thana comparable device containing [Ru(terpy)₂]⁰. Without being bound by anytheory, it is believed that this is attributed to the lower Φ of thereduced complex, which in turn yields a lower barrier for electroninjection. Referring again to FIG. 7, devices using [Cr(bpy)₃]⁰ produceless light emission than those with [Ru(terpy)₂]⁰.

In general, the performance of OLEDs having a relatively thick layer ofLWOM is superior to those with thin layers (i.e., <100 Å). This is incontrast to what was observed for more insulating materials such asinorganic salts and CuPc where only very thin layers of less than 50 Åare effective. Light emission from several devices differing only in thethickness of LWOM is shown in FIG. 9. As can be seen, the OLEDs havingthicker layers (89 or 185 Å) of [Cr(bpy)₃]⁰ produce several orders ofmagnitude more light than do those with only 37 Å.

It is believed that analysis of the current-voltage, i.e., I-V, curvescan provide understanding of the nature of the electron-injectionprocess in OLEDs. For example, in devices governed by trap-limitedelectron conduction in the Alq₃ layer, the power law equation I ∝V^(m−1) (where m is typically 7-9) describes the I-V relationship. Inthis case, the current is limited by transport characteristics of theAlq₃, rather than a barrier to injection at the interface with thecathode. For the OLEDs using metal-ligand complexes of the presentinvention, an exponential dependence of I on V is well obeyed. However,I-V curve does not fit to the power law relation satisfactorily. The I-Vperformance, in the useful operating range of 4-10 V, of the previouslydiscussed [Ru(terpy)₂]⁰ containing device (with PEDOT-PSS), along withthe exponential relationship I ∝ e^(cV) is shown in FIG. 9. Thissuggests that some OLED devices derived from metal-ligand coordinationcomplexes of the present invention are governed not by trap-limitedconduction in the Alq₃, but rather by an injection-limited mechanism.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. Althoughthe description of the invention has included description of one or moreembodiments and certain variations and modifications, other variationsand modifications are within the scope of the invention, e.g., as may bewithin the skill and knowledge of those in the art, after understandingthe present disclosure. It is intended to obtain rights which includealternative embodiments to the extent permitted, including alternate,interchangeable and/or equivalent structures, functions, ranges or stepsto those claimed, whether or not such alternate, interchangeable and/orequivalent structures, functions, ranges or steps are disclosed herein,and without intending to publicly dedicate any patentable subjectmatter. All references cited herein are incorporated by reference intheir entirety.

1. An electrode composition comprising: a current conducting material; and a heteroaryl-metal complex in contact with said current conducting material, wherein said heteroaryl-metal complex is of the formula: [M-(L)_(a)]_(m Y) _(n) wherein a is an integer from 1 to 6; m and n are absolute value of oxidation state of Y or [M-(L)_(a)], respectively; or if [M-(L)_(a)] is not charged Y is not present and said heteroaryl-metal complex is of the formula M(L)_(a); M is a metal; Y is a counterion; and each L is independently a heteroaryl moiety containing one or more coordinating heteroatoms.
 2. The electrode composition of claim 1 having work function of about 3.5 eV or less.
 3. The electrode composition of claim 1, wherein said heteroaryl-metal complex is of the formula M-(L)_(a).
 4. The electrode composition of claim 3, wherein a is an integer of 2 or
 3. 5. The electrode composition of claim 4, wherein M is a transition metal.
 6. The electrode composition of claim 5, wherein M is selected from the group consisting of Ru, Cr, Fe, Zn, Co, Mn, Cu, Os, Rh, and Ni.
 7. The electrode composition of claim 6, wherein M is selected from the group consisting of Ru and Cr.
 8. The electrode composition of claim 5, wherein L is a polypyridyl or phenanthroline moiety.
 9. The electrode composition of claim 8, wherein L is selected fiom the group consisting of optionally substituted 2,2′-bipyridyl, optionally substituted 1,10-phenanthroline, optionally substituted 2,2′,6′,2″-terpyridyl and a derivative thereof.
 10. The electrode composition of claim 8, wherein L is a polypyridyl moiety.
 11. The electrode composition of claim 10, wherein L is selected from the group consisting of 4,4′,5 ,5′-tetramethyl-2,2′-bipyridyl; 2,2′-bipyridyl; and 2,2′, 6′, 2″-terpyridy.
 12. The electrode composition of claim 1, wherein said current conducting material is a metal or a metal alloy.
 13. The electrode composition of claim 12, wherein said current conducting material comprises silver, gold or a mixture thereof.
 14. An electronic device comprising an electrode of claim
 1. 15. A composition comprising a metal or a metal alloy in contact with a heteroaryl-metal coordination complex, wherein said heteroaryl-metal coordination complex is of the formula: M-(L)_(a) wherein a is an integer from 1 to 6; M is a metal; and each L is independently a heteroaryl moiety containing one or more coordinating heteroatoms. 